Concreting wooden fence posts into clay soil

[jupiter77]

Hi, I'm building a lean to roof down side of my house. I'll be using 4x4 wooden fence posts to support the eaves side of the roof at a height of around 6.5ft. I've dug 2ft holes to set them in concrete. After about 1ft down the ground is very clay-y. Is it a good idea to put 2" of gravel at the bottom of the holes? I'm questioning as the clay will not drain well so am I just creating a 'well' at the bottom of the hole that water travelling through the topsoil will flow into and collect? Maybe its better to fill the hole completely with concrete (while making sure the end of the post is not sticking out/exposed at the bottom)?
But thinking some more, I'm wondering if this is a non-issue. There's no exposed soil/lawn within about 5-10 metres in any direction from the area of the posts, any rain will fall on adjacent roofs or patio/driveways with surface drains. So no rain will fall on the area immediately around the post and will much water realistically flow to the area underground?
Any advice appreciated, I'm paranoid about rotting posts!

Also as the roof is partially supported by the wall, and is not a fence, is the usual 3x post width hole diameter necessary, or can I get away with less, say 10 inches for 4" posts?

 

[blup]

Get good quality pressure treated posts from your local timber merchants, they will make a big difference, I have fence posts that have been in thirty years and held fences upright. Consider bolting on a concrete spur for added strength and longevity.

Blup

 

[jupiter77]

Thanks, yes I've got treated timber

 

[JohnD]

In this job, I strongly recommend a concrete pad cast into the ground, with a steel shoe, and the post fixed into the shoe above ground.

Buried timber will always rot, and eventually snap just above ground where the ideal combination of damp and air content occur.

You can make a simple box form around the top of the pad so the visible part looks small and neat. Slightly sloped or domed to throw off rain.

Example in the left corner of this lean-to shed.

Unlike a fence, a shed is not subject to strong sideways forces from winds trying to blow it over.

The concrete spur you can see is to raise a pre-existing fencepost off the ground, I incorporated it into the shed.

 

[Stivino]

The reason the posts snap at the top is because they have been set into the concrete, as opposed to through the concrete and into hardcore,which is the way they should be done.

 

[freddiemercurystwin]

They snap because the timber goes rotten.

 

[jupiter77]

Thanks for the replies.

What do people think of this video? It seems to agree with my instincts on using gravel at the bottom of posts in clay soils and the potential for creating a well for water to collect.

Anyway I've got some car underbody sealant on the posts (another debate - to paint the end grain or not?). I'm favouring not bothering with gravel at the bottom of the hole, but ensuring the base of the post is fully enclosed in concrete, not sticking out the end.

 

[freddiemercurystwin]

Concrete is not waterproof, if you want to avoid the post rotting then it cannot go into the ground, sure you can coat it in bitumen or you can stick a plastic wrap around the post (think someone makes these) where it enters the ground as that's where they fail, that might help to prolong it's life but really the solution is to sit it on a steel bracket sat on a concrete pad as already mentioned, though there are far more elegant solutions than the example posted. Gravel in the bottom won't make a jot of difference, clay doesn't drain anyway the water will just sit there.

 

[scbk]

You get different species of timber, and different types of treatment

https://www.thewpa.org.uk/_files/ugd/65ba63_9d31126c1648485197b53539998f7c40.pdf?index=true#:~:text=Use%20Class%204%20is%20for,or%20providing%20exterior%20structural%20support.&text=Preservative%20treatment%20provides%20wood%20with,treated%20wood%20is%20the%20same.

You can also add to it yourself with creosote, which is available for "professional use only" in minimum of 20litres

 

[jupiter77]

Posts are in now, without gravel and painted with underbody seal. Time will tell...